If they are working and paying tax then who cares if they are taking welfare. They are paying into the system after all. As long as they aren't completely freeloading, aren't jumping housing queues and bullshit like that then it's fine. And those are already things that are within most nations power to change as it is. If migrants are being given multi million pound properties in central London as some trustworthy publications suggest *dailymail egh* then that is a fault of the existing system. Stopping migration won't alter what is already a broken internal system.
Plus you have the nations with a falling population, Germany being the prime example, that need migration if they are going to maintain economic growth.
Like I said, it doesn't matter whether your in or out, migration will always exist and in all likelihood politicians, even if given more power, won't do jack shit about it because the overall benefits are too great as long as economic growth is the overall goal. Which for western capitalist nations, it is. The only way you'd probably change that is by voting in an extreme far right party.
Edit:
Fact is migration is the go to topic for those who want to vote out. I've barely heard any substantial arguments of any other kind bar some wishy washy bollocks about sovereignty. It's the topic that drums up the feelings of morons but is probably the least applicable topic to the EU debate. There are far bigger issues like TTIP which you just don't see discussed, even though they have far more bearing on our every day lives, that would swing my vote to leave. As it is I'm probably going to vote in because I'm sick to death of the migration discussion being the only real topic leave cling onto and there's so little information about other more concerning factors.